1. Field of the Invention
This invention relates to a method of forming an image by the use of a radiation, and more particularly to an image formation method utilizing the electroradiography which effects image formation by using gases ionizable by an ionization such as X-rays, ultraviolet rays, gamma rays or the like.
2. Description of the Prior Art
Image formation methods which use X-rays to ionize gases and effect the image formation have heretofore been disclosed in published German Pat. No. 1,497,093 specification (hereinafter referred to as DAS 1,497,093) and open German Pat. No. 2,258,364 specification (hereinafter referred to as DOS 2,258,364). For example, according to the technique disclosed in DOS 2,258,364, a film of insulative material for retaining a latent image thereon is inserted in a gas chamber provided by a gap portion defined by and between two electrodes. As the ionizable gases, gases of substances of atomic number 36 and up such as xenon and others are introduced at a high pressure to fill the gas chamber. These gases, when irradiated with X-rays, are ionized to produce electrons and positive ions, but in order to increase the absorption of the X-rays and provide a good sensitivity, the gases must be maintained at several tens of atmospheric pressures if the thickness of the gap portion is of the order of 2mm. Those electrons and positive ions produced from the gases upon their exposure to the X-rays through a body are attracted to the electrode portion by the action of the field resulting from the electrodes, whereby an electrostatic latent image is formed on the film. The technique disclosed in DAS 1,497,093 is approximate in principle to that described above, but the pressure of the gases used therein is close to the atmospheric pressure.
In the technique disclosed in DOS 2,258,364, gases at several tens of atmospheric pressures should preferably be used if the gap portion forming the gas chamber is of the order of 2mm, and this necessitates a high degree of skill in the choice of the gas chamber construction and the material therefor as well as in the handling of the high pressure gases. A solution to this problem would be to reduce the gas pressure in the gas chamber, which in turn would lead to the necessity of increasing the thickness of the gas portion forming the gas chamber. However, an increased thickness of the gap portion would reduce the image resolving power of the electrostatic latent image in the marginal region thereof. Such phenomenon of reduced resolving power will be described by reference to FIG. 1 of the accompanying drawings. FIG. 1(a) schematically depicts an electroradiography apparatus, in which a source of X-rays is designated by numeral 1 and high pressure gases such as xenon, krypton, radon, etc. are present in a gap portion 4 defined by and between electrodes 2 and 3. In the apparatus of FIG. 1(a), it is assumed that the distance from the X-ray source 1 to the electrode adjacent the X-ray source is 1.5 m and that the thickness of the gap portion 4 is 10mm instead of 2mm, because the gas pressure therein is selected to the order of five atmospheric pressures, instead of several tens of atmospheric pressures which should originally be the gas pressure level. In such case, when X-rays carrying a spot image is applied between the two electrodes, the high pressure gases will be ionized by the X-rays to produce charges and these charges will be attracted to an insulative film 5 (see FIG. 1(b)) disposed on the electrode 2, whereby an electrostatic latent image will be formed on the film. This completed electrostatic latent image, when seen, will be such that a region thereof which should form a spot image, which has existed on a diameter of 350mm, appears as a line of 1.17mm. More specifically, where the electrodes are planar and the gap portion defined by the electrodes is thick and if the X-rays are emitted from a single spot, there will occur a problem that the electrostatic latent image formed on the insulative film has a resolving power progressively reduced from the center region toward the marginal region.